The present invention relates to a method of controlling the pressure in an incinerator for incinerating municipal refuse, industrial waste, etc. by controlling the flow rate of exhaust gas.
In an incinerator for municipal refuse or the like, the pressure in the furnace must be constantly kept at a predetermined negative pressure from the viewpoint of safety. If the negative pressure is excessively high, the amount of leakage air at the furnace, an exhaust gas cooler, a gas treating device, an exhaust gas duct, etc. is high, whereby the amount of exhaust gas is high resulting in a large amount of electric power being consumed by an induced draft fan. Accordingly, it is necessary to regulate the pressure in the furnace to an appropriate negative pressure.
In general, the control of the pressure in an incinerator has heretofore been effected by using a simple control system such as that shown in FIG. 2, in which reference numeral 1 denotes an incinerator, 2 a gas cooler, 3 a gas treating device, 4 a remote-control exhaust gas damper for controlling the flow rate of exhaust gas, 5 an induced draft fan for suction of exhaust gas, and 6 a stack. The incinerator 1 is fed with fuel 7 and combustion air 8. Reference numeral 13 denotes leakage. The pressure in the incinerator 1 that is detected by a detecting element 9 is transmitted by a pressure transmitter 10 to a controller 12 including a PID controller 11, where it is compared with a furnace pressure set value to obtain a manipulated variable signal, and the remote-control exhaust gas damper 4, serving as a final control element, is controlled on the basis of the manipulated variable signal to thereby control the flow rate of exhaust gas.
Such a conventional control system is satisfactory for practical use in general combustion furnaces but not for incinerators designed for municipal refuse, which refuse varies greatly in both quality and quantity, because such incinerators have drastic, oscillatory and irregular variations in the furnace pressure in comparison with relatively stable furnaces such as heavy oil incinerators. Accordingly, it is difficult for a simple control system such as that described above to effect a stable control of the pressure in the incinerators for municipal refuse.
There is a prior art control system designed to cope with this problem, e.g., the one disclosed in Japanese Patent Public Disclosure (KOKAI) No. 61-49929 (1986) entitled "Furnace Pressure Control System", filed by the present applicant. FIG. 3 shows the arrangement of this furnace pressure control system. Referring to the figure, a controller 12 includes a first-order lag filter 15, a subtracter 16, a non-linear operator 17, a differentiator 18, a non-linear operator 19 and an adder 20. The differentiator 18 and the non-linear operator 19 constitute in combination a differential output circuit 21.
In the furnace pressure control system having the above-described arrangement, when the differential output circuit 21 is not employed, the pressure in the incinerator 1 is transmitted as an output PV.sub.0 to the first-order lag filter 15 by the pressure transmitter 10. The filter 15 filters out ripples to produce an output PV.sub.1. The subtracter 16 obtains a difference between the output PV.sub.1 and a set value SV in the PID controller 11 and delivers an output PV.sub.2, which is input to the non-linear operator 17. The operator 17 delivers an output PV.sub.3 with a gain selected in accordance with conditions, that is, whether SV&lt;PV.sub.1 or SV&gt;PV.sub.1.
More specifically, the gain that is selected when SV&lt;PV.sub.1 is larger than that when SV&gt;PV.sub.1.
The output PV.sub.3 is subjected to a PID operation in the PID controller 11 to deliver an output MV.sub.1, which is input to the adder 20 to deliver an output MV.sub.0. In this case, there is no input to be added to MV.sub.1. Hence, MV.sub.0 =MV.sub.1. With the output MV.sub.0, the remote-control exhaust gas damper 4 is controlled. However, since the gain is changed as described above, the value of the output MV.sub.0 is larger when SV&lt;PV.sub.1 than in when SV&gt;PV.sub.1. Accordingly, the operating speed of the remote-control exhaust gas damper 4, which is a final control element, is higher when SV&lt;PV.sub.1 than when SV&gt;PV.sub.1, thereby promptly suppressing the rise in the furnace pressure, and thus preventing it from becoming positive.
When the differential output circuit 21 is employed in the furnace pressure control system shown in FIG. 3, the non-linear operator 17 may not necessarily need to change the gain on the basis of the comparison between SV and PV.sub.1. The output PV.sub.0 is differentiated in the differentiator 18 to deliver an output y.sub.1, which is input to the non-linear operator 19. The operator 19 delivers an output y.sub.2 only when the differential value is positive. The output y.sub.2 is added to the output MV.sub.1 delivered from the PID controller 11 as a fundamental manipulated variable in the adder 20 to generate a corrected manipulated variable signal MV.sub.0, which is used to control the remote-control exhaust gas damper 4 serving as a final control element. An upward tendency of the furnace pressure is judged by the fact that the differential value is positive, and in such a case a larger manipulated variable is given to the final control element to increase the operating speed of the exhaust gas damper 4, thereby promptly suppressing the rise in the furnace pressure, and thus preventing it from becoming positive.
Recently, exhaust gas treatment has been improved. That is, it has heretofore been common practice to employ an electrostatic precipitator for exhaust gas treatment, whereas it has recently become common practice to employ a bag filter or wet-type treatment or to pass exhaust gas through a chemical-packed bed. Thus, the pressure loss is high in the recent treatment of exhaust gas. When the exhaust gas treatment is accompanied by a large pressure loss, it is likely with the method disclosed in Japanese Patent Public Disclosure (KOKAI) NO. 61-49929 (1986) that the induced draft fan 5 will transiently have a deficient capacity due to the delay in operation of the remote-control exhaust gas damper 4, resulting in an abnormally positive furnace pressure. If the furnace pressure becomes positive, the combustion gas leaks out of the system, which is unfavorable for the working environment.